docs(master): add metadata service design summary (#951)

[yangcx000]

Summary

Add a Chinese review document that summarizes Curvine metadata-service behavior and proposes an evolution path toward a stateless MDS with metadata storage separated by WriteType.

The document distinguishes current implementation facts from proposed design directions. It is intentionally submitted as a draft for community discussion; the per-operation protocol design in Chapter 4 remains to be completed after the high-level direction is reviewed.

Issue Describe / Design

Related to #951 and #1321.

The current Master keeps CacheMode soft-state metadata and FsMode/native authoritative metadata in the same in-memory tree, lock domain, RocksDB store, and Raft journal. This couples request-serving capacity, metadata capacity, and persistence policy even though the two metadata classes have different consistency and durability requirements.

The document:

• summarizes Curvine use cases, clients, and metadata pressure patterns;
• derives metadata requirements from CacheMode and FsMode product semantics;
• proposes separating soft-state attribute caching from authoritative POSIX metadata;
• describes a stateless MDS computation layer with pluggable KV backends;
• outlines schema, transaction, routing, recovery, and inode-addressing requirements;
• records open design work for per-operation interaction and cross-shard rename.

This PR proposes a discussion document only. It does not change runtime behavior or commit the project to a specific storage backend.

Changes

Module / File	Change	Impact on existing behavior
metadata-service-summary.md	Add the MDS current-state summary and proposed metadata-service architecture	Documentation only; no runtime behavior change

Test verified

Test case	Result	Notes
git diff --no-index --check /dev/null metadata-service-summary.md	PASS	No whitespace errors
Manual document review	PASS	Checked headings, repository paths, terminology, and local-only references

Dependencies

• Related PRs: None
• Related issues: [FEATURE] Make the Master stateless to enable horizontal scaling, with metadata stored in an external distributed database #951, [FEATURE] Shorten the master FsDir metadata write-lock critical path #1321
• Blocks / blocked by: Community design review before implementation planning